A major challenge facing the AIDS field is the eradication of the human immunodeficiency virus type-1 (HIV-1). This effort will require the development of new therapies that specifically target the provirus residing within infected cells in a status of non-productive infection. In turn, advances toward that goal will require a quantum leap in our knowledge and understanding of the biology of HIV-1 latency. However, all the current systems to study HIV-1 latency remain severely inadequate for that task. This application seeks to employ a novel in vitro model to advance our knowledge about the distinctive features of latently infected cells. Viral latency is an important problem in clinical practice, and represents a major obstacle to virus eradication. The pool of latently infected cells is established during primary infection and persists throughout the course of the disease. As a dormant provirus, HIV-1 is invisible to cellular and humoral immune responses, and is refractory to anti-retroviral drugs. The latent reservoir archives drug-resistant viruses that can re-emerge in the context of inadequate therapy or poor compliance, and is responsible for the rebound viremia observed following structured therapy interruption. Viral latency may also affect the natural history of HIV-1 infection and disease progression by playing a role in virus transmission and coreceptor switch. Whereas CCR5-tropic HIV-1 strains are responsible for virus transmission during primary infection and persist throughout disease, emergence of CXCR4-tropic strains that occurs in about half of all cases is associated with faster disease progression. The hypothesis that the two variants might display different propensities to establish and re- emerge from latency could help explain changes in coreceptor usage observed during HIV-1 infection. So far, the lack of a valid and reliable cellular system to study HIV-1 latency has hindered progress toward the understanding of its biology and the development of targeted therapeutic strategies. We have recently described a new in vitro culture system to investigate the induction, maintenance and reactivation of HIV-1 latency. Our cellular system recapitulates the events of primary and secondary antigen-driven immune response in which CD4+ T cells are activated with dendritic cells and antigen, infected in vitro with HIV-1, and then brought back to quiescence through a resting phase in the presence of interleukin-7. We have demonstrated that these in vitro-generated latently infected cells lack expression of activation markers;do not undergo cellular proliferation and do not sustain viral replication. Finally, we have shown that all these activities resume promptly following secondary antigen stimulation. Our system is amenable to answer key questions and to provide new insights into the nature of latently infected cells. Therefore, the present application proposes to determine the gene expression profile of in vitro- generated latently infected cells isolated by fluorescence activated cell sorting. Moreover, we propose to test the hypothesis that CCR5- and CXCR4-tropic HIV-1 strains show different propensities to establish and re- emerge from latency. PUBLIC HEALTH RELEVANCE: The human immunodeficiency virus type-1 (HIV-1) establishes a status of non-productive infection known as latency. As a dormant provirus, HIV-1 is invisible to immune responses, and resistant to anti-retroviral drugs. Thus, latency is an important problem in clinical practice. Eradication of HIV-1 will require the development of new therapeutic strategies that target specifically latently infected cells. So far, progress in that direction has been hindered by limited knowledge of HIV-1 latency. In this application we propose to make use of an in vitro cellular system we have developed recently to gain new insight into the distinctive features of latently infected cells, and the mechanisms that regulate induction, maintenance and reactivation of HIV-1 latency. The new knowledge we expect to draw from these studies could lead to the design of new therapies aimed at the pool of latently infected cells, and to virus eradication.